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Then perhaps you'd like to tell us why it matters.
– Barrie EnglandJun 28 '12 at 17:52

Okay... so I think there's importance to it because on the radio, like NPR, they're always saying "the historical record tell us..." and so I just think, "Why aren't they saying historic? How do they know that they should be saying historical instead? They must be right... they're ON NPR..." Then, frustrated and down trodden, I turn off the radio, non the wiser to grammatical usage or the important message of the"historical" record. Sighhh. So that's why I'm asking you guys. So I can finally hear the story with a clear, distraction-free mind!
– OnWordsJun 28 '12 at 22:33

Look them up in a dictionary. In some cases, the meaning is nearly the same, but in others, for example "literate" and "literary", there can be huge differences of meaning.
– Peter Shor Jun 29 '12 at 3:48

2 Answers
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I don't know that there's any general rule about how you would interpret adjectives that sound similar or have common roots.

In the examples you give, "historic" means that an event is important; "historical" means something related to history. So an "historical novel" would be a story about history, that is, a story set in the past. An "historic novel" would be a novel that is great and important. One might very well write a boring historical novel, i.e. a historical novel that is not historic; or a historic novel set in the present, i.e. a historic novel that is not historical.

"Instructive" refers to something that imparts insight or knowledge. "Instructional" describes something intended to be used for teaching. If you say, "Sally's over-reaction to the news was very instructive", you mean that you gained an insight into Sally's psychology from her behavior. If you say, "This book is instructional", you mean that it is intended to teach something, as opposed to being for entertainment or some other purpose.

In general, if you see such similar words, the best thing to do is look them up in a dictionary or at least study the context in which you found them to understand the difference in their meanings.